Are mental illnesses biological diseases

Are mental illnesses biological diseases? Sometimes a mental illness can be caused by a chemical imbalance I believe in that, but I don’t necessarily believe that a mental illness can be passed down from generation to generation through genetics, it just doesn’t seem like something feasible, the complexity of the mind is extreme I doubt that something as complex as a mental illness can be passed down from father to son. Lot’s of people believe this realistically I don’t, I think a lot of people believe this simply because there is history of mental illness in the family so therefore supposedly there is a higher likelihood of you having it because of biological defects. I don’t realistically believe that, I think it just has to do with overall upbringing, your great, great grandpa was a horrible person he was rude 24 hours a day, he never showed your grandpa any actual real love or respect and therefore later on in life when your grandpa got older he was diagnosed with clinical depression.